Stabilized rotary blades

ABSTRACT

Projections near the tips of high speed rotary blades are actuated by centrifugal force to ensure uniform spacing between the blade tips as a means of reducing buzz-saw noise.

United States Patent [191 Tracy STABILIZED ROTARY BLADES [75] Inventor:Alfred C. Tracy, Los Angeles, Calif.

[73] Assignee: McDonnell Douglas Corporation,

Santa Monica, Calif.

[22] Filed: Oct. 30, 1972 [21] Appl. No.: 301,827

I521 U.S.Cl ..4l6/196,4l6/190,416/191 [51 j Int. Cl. F0ld 5/22 [58]Field of Search 416/190, 191, 196,

[56] References Cited UNITED STATES PATENTS Anxionnaz 416/196 X [451Nov. 13,1973

2,914,299 11/1959 Mitchell 416/196 UX 3,104,093 9/1963 Craig et a1.416/210 FOREIGN PATENTS OR APPLICATIONS 1,426,798 3/1969 Germany 416/196Primary Examiner-Everette A. Powell, Jr. Att0rneyWa1ter .1. Jason et a1.

[57] ABSTRACT Projections near the tips of high speed rotary blades areactuated by centrifugal force to ensure uniform spacing between theblade tips as a means of reducing buzz-saw noise.

3 Claims, 5 Drawing Figures STABILIZED ROTARY BLADES BACKGROUND OF THEINVENTION In modern high speed turbo engines for aircraft, the tendencyis to use turbofan engines with as high an engine bypass ratio aspossible. This calls for increasingly larger fan blades. Through thenecessity of damping and because of other stress problems, the rootsupport structure of the blades must be loose. Dimensional variations inspacing at the tips of the blades thus may become quite large, producinguneven acoustical pulses with a fundamental or lowest frequency startingwith the first rotational frequency and all harmonics thereof.

The highest frequency would be the frequency of a.

blade passage over a point on its periphery and in the case of anaircraft engine would be on the order of 2,000 cps. In a perfectsituation, in which there were no buzz-saw noise, there would be novariation in the blades characteristics, such as angle of attack, tipspacing, camber, contour, etc. By stabilizing the tips of the blades inaccordance with the present invention, there is great improvement in thereduction of buzz-saw noise generated by the rotor tips. Theoretically,blades spaced perfectly and of identical manufacture will produce noharmonics below the 38th in the case of the DC-l jet engines and it isthe fifth through the 20th harmonic that causes the buzz-saw noise.Attempts have been made to reduce the intensity of this noise throughtreatment in the cabin side walls and attempts have been made to reducethe compressor whine and the turbine whine by perforated linings in thein-take and the exhaust. While these treatments affect the harmonics ofthe buzz-saw noise, they do notreduce the lower frequency buzz-saw noisesource through the stabilization of rolling deflections of each bladeand through the control of spacing between the blades.

SUMMARY OF TI-IE PRESENT INVENTION In accordance with the presentinvention, turbine blades are designed with side arms near thethreequarter span position. These arms are deflected slightly inwardlytoward the rotor hub and utilizes the centrifugal force that occursduring the normal running of the engine to bend the arms in an elasticdeflection such that adjacent blade arms come in contact with each otherand thus limit the spacing between the blades. These elasticallydeflected arms stabilize the dimensions between the blades and will alsoadd to the damping of each blade, reducing the blade vibration anappreciable amount. A bend relief-at the root of the arm may be desiredin some instances forgreater flexibility and greater deflection underthe centrifugal loading.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view withparts broken away to more clearly show the rotor blades in an aircraftturbofan engine;

FIG. 2 is an enlarged view of a portion of the blades showing theextended arms near the blade tips;

FIG. 3 is a front view enlargement showing the arms in normal positionand their deflection position due to centrifugal force;

FIG. 4 is a view similar to that of FIG. 2 wherein the arms have beenextended by centrifugal force upon rotation of the blades; and

FIG. 5 is a plan view taken along the line 5-5 in FIG. 4.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT Reference is now made toFIG. 1 wherein there is shown the housing 10 about an aircraft engineand within which the blades 12 of the turbine are rotatably mounted upona rotor or hub 14. The housing 10 typically may have a diameter 8 or 9feet and the blades may have a length on the order of 3 or 4 feet. Theseblades should be as light as possible, yet must be free from destructivevibration or flutter. These blades are subjected to large twisting andbending moments. Because of stress reasons the blades are looselyattached to the rotor within hub 14 such that considerable mechanicalclatter may be heard when the engines are being shut down such as whenthe aircraft has taxied to the unloading station at the end of theflight. However, it is not this clatter that the present inventionovercomes but it is the uninherent, uneven spacing of the blades as theypass a point on the periphery of the housing that generates the lowfrequency sawtooth noise when the engines are operating. For example, ifthere were 39 blades and the rotational speed were cycles per second,then the blade passage frequency would be 2,340 cps. Any variation inangle of attack or tip spacing, camber, contour, or other characteristicwill produce a buzz-saw noise within a range of from 200 cps to 1,000cps. Just forwardly of the blades are engine housing extensions 16 and18 having an outer surface compatible with the outer surface of thehousing 10 and inner surfaces compatible with the inner surface ofhousing 10. These sections preferably are made of highstrength-to-weight panels to form rings having a honeycomb or cellularmaterial sandwiched between the inner and outer faces 22 and 24. Byproviding a plurality of openings 26 within the inner surface 24 some ofthe sound waves may be damped within the inner cavities. This particularapproach is more clearly set forth in co-pending patent application Ser.No. 286,457 filed Sept. 5, 1972 by Arnold W. Guess and Guenter M.Schindler and forms no part of the present invention except incombination therewith.

Referring now to FIG. 2 there is shown a plurality of blades 12 havinglaterally extending arms 28 extending from both sides of the .bladeswithin the outer onefourth of the length of the blade 12. These arms areof a generally flat plate-like structure having an outer edge surface 30extending relatively parallel to the surface of the blade 12 from whichthe arm '28 extends. The outer edge 30 of arm 28 is bent downwardlysomewhat in the direction of the hub and in this position the adjacentanns will have a small gap therebetween.

A front enlarged view may be seen in FIG. 3 wherein adjacent blades 12and 12A are shown spaced apart but there is a clearance or a spacing onthe order of 0.001 to 0.005 inch between the free edges 30 and 30A ofthe adjacent arms 28 and 28A when in this inwardly directednon-deflecting position. The elasticity of the arms 28, 28A at theirjuncture with the blades 12 and 12A is such that there may be anoutwardly directed deflection on the order of 0.015 to 0.020 inch due tocentrifugal force when the blades are rotating at conventional speed.This outward deflection moves the arms 28, 28A to their dotted linepositions 28D and 28AD with the letter D designating its deflectedposition. These arms 28 and 28A may have a bend relief 32 on thecompression side and a bend relief 34 on its tension side of thejuncture between the arms and the blades. The extent of this bendrelief, of course, depends upon the elasticity of the material used andthe amount of deflection desired for the amount of centrifugal forcedetermined by the rotation of the blades. It is conceivable that in somecases such bend reliefs may not be necessary and in fact fillets may beused.

In FIG. 4 there is shown a fan of blades 12 having a plurality of arms28 extending therefrom about a circumference near the outer ends 36 ofthe blades. These arms 28 are shown extended outwardly and innearabutting relationship due to centrifugal force as the blower bladesrotate. These arms 28 have filleted connections to the blades since abend relief is not necessary in this particular application.

Reference is made to the plan view in FIG. 5 taken along the line 5-5 inFIG. 4. Inasmuch as the turbine blades 12 are angularly orientedrelative to the axis of the hub 14, the arms 28 also are angularlyoriented such that the outer edges 30 are generally parallel with thesurface of the blades as previously mentioned until centrifugal forcedeflects the adjacent arms 28, 28A into abutting contact there is a gapbetween the adjacent outer edges 30 and 30A. This gap, of course, closeswhen the rotors reach conventional speed and centrifugal force causesthe outward deflection of the arms.

Having thus described an illustrative embodiment of the presentinvention, it is to be understood that other modifications andvariations will occur to those skilled in the art and it is to beunderstood that these deviations from the illustrative embodiment are tobe considered part of the present invention as claimed.

I claim:

1. In combination with a turbofan engine having a plurality of bladesattached to a rotor hub and a housing having a circumferential innersurface in close radial proximity to the tips of said blades,

means for reducing noise caused by irregular spacing of said blade tipsand thus varying the frequency of blade tip passage over a point on theinner surface of the housing about the periphery within which said bladetips rotate,

said means comprising side arms extending laterally outwardly from thesurfaces of said blades and radially inwardly toward said hub,

said arms having outer edge surfaces positioned radially inwardly fromthe connection of said arms to said blades,

said arms deflecting radially outwardly due to centrifugal force whensaid blades are rotated at a preselected frequency, the outer edgesurfaces of adjacent arms on adjacent blades abutting each other whensaid blades rotate at said preselected frequency.

2. The combination as in claim 1 wherein said arms are of equal lengththereby to cause equal spacing of said blade tips when said blades arerotated at said preselected frequency.

3. The combination as in claim 1 wherein said outer edge surfaces aresubstantially parallel to the surfaces of said blades in the armthickness direction and in the arm length direction to define abuttingplanes parallel to said blade surfaces.

1. In combination with a turbofan engine having a plurality of bladesattached to a rotor hub and a housing having a circumferential innersurface in close radial proximity to the tips of said blades, means forreducing noise caused by irregular spacing of said blade tips and thusvarying the frequency of blade tip passage over a point on the innersurface of the housing about the periphery within which said blade tipsrotate, said means comprising side arms extending laterally outwardlyfrom the surfaces of said blades and radially inwardly toward said hub,said arms having outer edge surfaces positioned radially inwardly fromthe connection of said arms to said blades, said arms deflectingradially outwardly due to centrifugal force when said blades are rotatedat a preselected frequency, the outer edge surfaces of adjacent arms onadjacent blades abutting each other when said blades rotate at saidpreselected frequency.
 2. The combination as in claim 1 wherein saidarms are of equal length thereby to cause equal spacing of said bladetips when said blades are rotated at said preselected frequency.
 3. Thecombination as in claim 1 wherein said outer edge surfaces aresubstantially parallel to the surfaces of said blades in the armthickness direction and in the arm length direction to define abuttingplanes parallel to said blade surfaces.